I have used Instacart a LOT now. More than 30 times (evidently I am a glutton for punishment or actually crazy expecting the service to miraculously improve somehow). Save yourself. Just go to the damn store, you'll be happier in the long run.
When I first used Instacart, the shoppers were fairly good about communicating changes. My orders were pretty solid. To be fair, I have had one or two shoppers in my long history who were pretty awesome. Apparently they have left, because the shoppers that work now either can't or don't read. Almost every order I have placed as of late has had MAJOR problems (like replacing $8 almond butter with a $27 brand. And yes, I was clear about not wanting to spend more than $8). I order 1/2 pound of lunch meat and end up with 2 pounds, $20 later. Oh - and you want to actually RECEIVE the things you order? Pish-posh! Instacart will charge you for items that don't get delivered. Almost every order is missing something. Even though I am very, very specific about what I want (or don't want), shoppers somehow manage to mangle my orders. Although the shoppers are supposed to offer replacement options, they rarely do. THIS SERVICE IS TERRIBLE.
SAVE YOURSELF.